In such a television receiver the sound intercarrier signal is produced by the mixer circuit, this sound intercarrier signal being a signal whose frequency is modulated with the audio signal, the frequency of the carrier thereof being equal to the difference between the frequencies of the two carriers. It is furthermore affected with an unwanted amplitude modulation. In order to obtain a sound intercarrier signal which is free from amplitude modulation, it is, in known television receivers, first amplified and thereafter applied to a limiter circuit. The limited signal has a substantially constant amplitude and is applied to a frequency demodulator for producing the demodulated audio signal.
It has been found, however, that the limiting circuit introduces a phase modulation in the sound intercarrier signal. As the frequency modulation is actually a kind of phase modulation, this signal limitation means a distortion of the demodulated audio signal obtained. It will therefore be obvious that for an audio reproduction which satisfies high quality requirements it is desirable that the amplitude of the sound intercarrier signal be kept substantially constant in a different manner. The mixer circuit which produces the intercarrier signal can indeed be designed in such manner that the amplitude modulation it introduces will be small. However, the mixer circuit then behaves as a limiter and causes a phase modulation which cannot be eliminated anymore.
It should be noted that United Kingdom Pat. No. 562,702 describes a circuit for a receiver of frequency-modulated signals in which a disturbing amplitude modulation is reduced because the detected amplitude modulation is multiplied by the frequency-modulated signal of audio frequency, which is affected with this amplitude modulation. The improvement is however increased as the modulation depth of the amplitude-modulated signal is smaller, so that it is of no use for the sound section in a television receiver because of the fact that the disturbing amplitude modulation is partly caused by the picture carrier signal, which has a modulation depth of very high values, even up to 100%. In addition, the known circuit produces an envelope whose frequency is double the frequency of the disturbance and which is difficult to eliminate.